A known method for measuring phosphor temperatures is based on the principle that certain phosphors fluoresce (i.e., emit light) when irradiated with ultraviolet light, and the characteristics of the emitted light vary with temperature. For example, if the phosphor is irradiated with a pulsed light source, the amplitude of emitted light following an excitation pulse diminishes with a decay-time constant that varies with temperature. To determine the surface temperature of an object utilizing this principle, a phosphor is deposited on the object, the phosphor is allowed to come to at least approximately the same temperature as the surface of the object, and the phosphor is then irradiated. The emitted light is detected and the decay-time constant is determined from the detected light. By comparing the measured decay-time constant to tabulated data giving the decay-time constant of the phosphor at various temperatures, the surface temperature of the object is determined, at least approximately. A general discussion of temperature measurements using fluorescing phosphors (also called thermographic phosphors) is contained in the article "Thermographic-Phosphor Temperature Measurements: Commercial And Defense-Related Applications" by Bruce W. Noel, et al., Instrum. Soc. Am. Paper No. 94-1003, 1994.
Conventional methods for determining the decay-time constant from detected light are generally complicated and require expensive circuits. One method, as described in U.S. Pat. No. 5,107,445, for example, uses digital processing techniques. The detected light from a fluorescing phosphor is converted to digital data and then curve-fitting techniques are used to determine the decay-time constant. Another method, as described in U.S. Pat. Nos. 4,752,141 and 4,652,143, for example, measures the time it takes for the amplitude of emitted light to decrease by a predetermined proportion. In this method, the signal level of emitted light is detected at a predetermined time interval after the excitation pulse to establish a baseline level. A counter is started when the baseline level is established. The baseline level is divided by the natural logarithmic base (e) to calculate a target signal level. When the level of emitted light falls to the target signal level, the counter is stopped. The time taken for the emitted light to decrease from the baseline level to 1/e times the baseline level, as indicated by the counter, is proportional to the decay-time constant.
These conventional techniques provide an exact value for the decay-time constant. However, these techniques require acquisition of data at multiple signal points and mathematical processing of data to perform curve-fitting or to calculate a target signal level. The circuits required for acquisition of the data and processing of the acquired data are expensive. Also, the process of acquiring and processing the data is time-consuming. Thus, although the conventional techniques provide accurate results, they are costly and do not provide results in real time.